The inner walls of any metallic body are free of charge and electrostatic fields. Therefore, if a charged particle external to a metallic cup hits the inside of the cup and is neutralized there, the accumulated charge will flow to the outer surface of the cup. This implies that it is possible to achieve a very high charge state of the cup by depositing charge on the inside of the cup, because no potential needs to be overcome by the approaching charge. This is the working principle of a Faraday cup detector. A charged particle beam enters the cup. The particle collides with the cup wall and is neutralized as the charge is transferred to the cup. In the case of a charged particle, the now neutral atom (or molecule) may leave or stay in the cup, depending on the sticking coefficient and cup temperature. At some point in time, before the charge can leak away by other means, the charge accumulated on the cup is measured by draining away the charge through a suitable circuit.
In practice, incoming particles with energies above approximately 300 eV may invoke sputtering on the cup surface during the collision with the cup wall. In this case secondary electrons or ions are created. These secondary charged particles may leave the cup, thus altering the net balance between the charge accumulated on the cup and the incoming charge flux. These secondary particles leave the surface of the cup with only low energy and thus can be retained in the cup by creating a retarding electric field with a low voltage. Such a low potential has little effect on the incident charged particle beam. Placing a suppressor grid or electrode with an appropriate voltage in front of the entrance to the cup typically creates the retarding electric fields in Faraday cup designs. These designs trade transmission into the cup for better retention of the charge that enters. The trade-off is necessary because of the difficulty in manufacturing cups by standard means with small opening cross-sectional dimensions and large depths that would allow for effective use of simple biasing.
In combination with conventional electrometers, well-designed Faraday cups can measure charged particle currents down to I=10−14 A, representing the charge on about 50,000 ions. Therefore, Faraday cups are not as sensitive as electron multipliers or microchannel plate detectors, which have single charged particle counting capabilities. The Faraday cup's main advantages include its extreme robustness and reliability, and its linearity and capability for measuring absolute ion currents. In addition, Faraday cups are charge-integrating devices that offer the potential for use in applications requiring the capturing of charge with ratios of the time charge is measured to the time available (duty cycle) approaching unity. Unlike devices depending upon charge cascades for gain, the operating principle of Faraday cups does not require high voltages. As a result, Faraday cups will not catastrophically break down, or induce spurious ionization, if the operating environment is not a high vacuum. In fact, they work independently of the vacuum conditions of the experimental layout and can even be used under atmospheric conditions. These features make Faraday cups an integral part of many instruments that require charged particle detection under less than ideal conditions.
Faraday cup detector arrays (FCDA) have been developed to measure the spatial distribution of ions or electrons in ion implantation applications (R. B. Liebert, “Method and Apparatus for Ion Beam Centroid Location,” U.S. Pat. No. 4,724,324, Feb. 9, 1988; M. Berte et al., “Device for Quantitative Display of the Current Density Within a Charged-Particle Beam”, U.S. Pat. No. 4,290,012, Nov. 5, 1979; S. Okuda et al., “Charged-Particle Distribution Measuring Apparatus”, U.S. Pat. No. 4,992,742, Nov. 15, 1989; V. M. Benveniste et al. “Ion Beam Profiling Method and Apparatus”, U.S. Pat. No. 5,198,676, Mar. 30, 1993. C. O'Morain, et al., “Large Diameter Plasma Profile Monitoring Using Faraday Cup Arrays,” Meas. Sci. Tech., Vol. 4, pp. 1484–1488, 1993; N. Natsuaki, et al., “Spatial Dose Uniformity Monitor for Electrically Scanned Beam,” Rev. Sci. Instrum., Vol. 49, No. 9, pp. 1300–1304, Sep. 1978.). Such a measurement has a resolution dependent on the finite size of each cup, the distance between adjacent cups in any dimension, and the width of the insulator between cups. A wide range of sizes can be, and have been, realized. Typically, designs do not consider ease, cost, and speed of manufacture, since they are for specialized applications, such as measuring beam profiles in experimental apparatuses or very high-cost electron microscopes.
Monitoring spatial distributions of charged particle beams is possible with several technologies, such as position-sensitive, microchannel-plate detectors (MCP), MCP-phosphorous screen units, or charge-coupled devices (CCD). These technologies are tailored to high sensitivity and provide the ability to count even single ions. However, they all lack linearity, ruggedness, and their amplification characteristics degrade over time. Furthermore, these devices cannot measure absolute ion currents if they are not particle counting, and are of only limited use in poor vacuum conditions, as found in the next generation of miniaturized mass spectrometers, such as portable or spacecraft-based instruments (M. P. Shiha, et al., “Development of a Miniature Gas Chromatograph—Mass Spectrometer,” Anal. Chem. Vol. 63 (18) pp. 2012–2016 (1991)). Finally, these solutions are cost intensive.
Faraday cup arrays have been developed for ion beam profiling purposes, but have been too large to be of use if high resolution is desired. (See above references.) They have been designed to provide spatial profiling of intense currents of a single type of charged particle. Device designers have given little or no attention to the requirements of measuring multiple ion currents covering a wide dynamic range beginning at low intensities and requiring high spatial resolution, such as would be seen in many mass spectrometric applications. Neither have designers been concerned with the speed of reading the array. Because applications have been specialized, present devices have not combined the previous requirements with the additional ones of ease, cost, and speed of manufacture.
The present invention addresses the key features that must be present in order for such a detector array to be useful and practical in applications other than the measurement of intense ion or electron beams, such as use as a detector in a Herzog-Mattuch type of mass spectrometer. The FCDA itself must have a fine pitch, typically of less than a millimeter from cup to cup, and it must be scalable up to several hundred cups in a linear array. The cups must intercept as much as possible of the incident charged particle beam, producing a high fill factor (ratio of sensitive detection area to the total area of the detector array). The cups and their interconnections to the electronic circuitry must be low leakage paths with controlled parasitic capacitances, with particular attention paid to minimizing the cup-to-cup capacitance, which increases as the overall array is miniaturized. The cups must also exhibit a high aspect ratio, being much deeper than wide in order to properly trap incident ions and suppress the emission of secondary electrons. Further, the FCDA must be tightly integrated with the electronic multiplexing unit (MUX) to produce a system that can be integrated into a vacuum chamber with only a minimal number of electrical feedthroughs. The multiplexing circuitry should be placed very close to the FCDA itself to minimize interference problems and maximize the signal-to-noise ratio (SNR). Crosstalk between cups and switching artifacts must also be reduced to a minimum in order to achieve full low-noise, high-sensitivity multichannel detection.
The present invention provides a charged particle beam detection system that includes a Faraday cup detector array (FCDA) and a tightly integrated electronic multiplexing unit (MUX). The FCDA of the invention has a variety of embodiments, several of which utilize modern microfabrication techniques and materials to realize all of the above features in a compact and economical design.